During operation of a gas turbine engine using a multi-stage axial compressor, the turbine rotor is turned at high speed by the turbine so that air is continuously introduced into the compressor, accelerated by the rotating blades and swept rearwards onto an adjacent row of stator vanes.
Inlet guide vanes are used to channel air into the compressor and the stator vanes correct the deflection given to the air by the rotor blades and present the air at the correct angle to the next stage of rotor blades.
It is known to provide variable inlet guide vanes (VIGVs) and variable stator vanes (VSVs) which have an adjustable angular deflection.
The manner of operation of a known variable vane system is described with reference to FIG. 1 which shows a cut-away side view of part of a compressor section of an aircraft gas turbine engine.
In FIG. 1 there is shown the compressor section 10 of an aircraft gas turbine engine. Sets of vanes 14 are circumferentially mounted on a tubular casing 12 about the central axis of the compressor section. A corresponding set of rotor vanes 16 is mounted downstream of each set of vanes 14. Each vane 14 terminates at the casing 12 in a stem 18 pivotable in a bush bearing 20 on the outside of the casing, the end of the stem 18 extending beyond the bush 20.
Located externally of the casing 12 and adjacent each set of vanes 14 are actuator rings 22 (also known as unison rings) extending circumferentially around the tubular casing 12. The vane stem 18 of each vane is connected to the corresponding actuator ring 22 by means of an actuating lever 24. One end of the actuating lever 24 is clamped to the end of the vane stem 18 by means of a bolt 26 so that there is no relative movement between the stem and the lever. The other end of the lever 24 is connected to the actuator ring 22 by a pin 28 which is pivotable in a bush bearing located in the actuator ring.
The actuator ring 22 is arranged so that it may be rotated in a circumferential direction about the central axis of the compressor section, for example, in either direction of arrow 9. Consequently, rotation of the actuator ring 22 will, by means of the actuating levers 24, cause rotation of each vane 14 about its own axis and thus enable the vanes 14 to assume pre-determined angles of incidence to the incoming air.
The known prior art systems are designed to effect a uniform angular deflection of all vanes 14 in a set upon rotation of the actuator ring 22. This uniform angular deflection has been found to result in engine vibrations which can causes damage and wear on the engine. Furthermore, the uniform angular deflection is unable to counter pressure variations in the air flow through the compressor. Another problem with the known systems is that angle variations can occur as a result of slack in the fittings in the unison ring and this can lead to pressure variations and, consequently, a reduction in the compressor efficiency.
There is a desire to provide an actuation arrangement that allows for individual deflection of a variable vane (for example, a VIGV or a VSV) to effect non-uniform deflection or to correct undesired non-uniformity introduced by slack in unison ring fittings across a set of variable vanes to reduce gas turbine engine vibration and to accommodate variations in the pressure of the air flow through the compressor.